In the past, a retract mandrel mill has been used to produce a seamless tube by a rolling process. Among prior arts, instances of using a retract mandrel mill are included in Patent Literatures 1 to 4.
[Configuration of Retract Mandrel Mill]
FIG. 1 is a configuration diagram of a conventional retract mandrel mill. As shown in FIG. 1, the retract mandrel mill includes a mandrel mill 10 which is a main rolling machine, and an extractor 20 which has the function of extracting a mandrel bar. In FIG. 1, the direction in which a tube blank 30 is rolled is indicated by an arrow A direction (hereafter, simply referred to as “rolling direction”).
The mandrel mill 10 includes a mandrel bar 11 and a plurality of rolls 12. A restraint mechanism 13 is provided at the end area on the entrance side (upstream in the rolling direction) of the mandrel mill 10. The mandrel bar 11 during a rolling operation advances in the rolling direction while being retained by the restraint mechanism 13, and retreats by the action of the restraint mechanism 13 after the rolling is completed.
The extractor 20 is disposed at the exit side (downstream in the rolling direction) of the mandrel mill 10 in series with the mandrel mill 10. The extractor 20 includes a plurality of rolls 22 in a housing 21.
[Method for Rolling Tube Blank]
When the tube blank 30 is rolled as the starting material for a seamless tube, the tube blank 30 is inserted with the mandrel bar 11 in the mandrel mill 10 and is rolled by means of the mandrel bar 11 and rolls 12. The mandrel bar 11 advances together with the tube blank when the tube blank 30 is rolled, and retreats to an initial position by the action of the restraint mechanism 13 after the rolling has ended.
Since the tube blank 30 rolled by the mandrel mill 10 is forced by the rolls 22 of the extractor 20 to advance in the rolling direction and the mandrel bar 11 is subject to the force exerted by the restraint mechanism 13 in the direction opposite to the advancing direction of the tube blank, the tube blank 30 can be separated from the mandrel bar 11. This operation is called as stripping.
In order to prohibit the mandrel bar 11 from intruding into the extractor 20, it is necessary to arrange that the distance between the mandrel mill 10 and the extractor 20 is no less than an amount that is obtained by an expression: (speed of mandrel bar)×(rolling time in the final roll of the mandrel mill). Since the rolling time in the final roll of the mandrel mill is proportionate to the length of the tube blank to be rolled in the mandrel mill, the distance between the mandrel mill 10 and the extractor 20 is proportionate to the speed of the mandrel bar and the length of the tube blank to be rolled in the mandrel mill.
In a conventional retract mandrel mill, the distance between the mandrel mill 10 and the extractor 20 is set according to the maximum length of the tube blank 30 to be rolled in the mandrel mill. Both of the mandrel mill 10 and the extractor 20 are fixedly disposed so that the distance between the mandrel mill 10 and the extractor 20 is not adjustable.
FIG. 2 is a diagram to illustrate a state where a tube blank, which is shorter than the distance between the mandrel mill and the extractor, is rolled in a conventional retract mandrel mill. FIG. 2A shows a state of rolling procedure at a mandrel mill, FIG. 2B shows a state where stripping is performed by using an extract fork, FIG. 2C shows a state where the tube blank after being rolled in the mandrel mill is moved by the mandrel bar, and FIG. 2D shows a state where the overlap between the mandrel bar and the tube blank is reduced.
When a tube blank 30 which is shorter than the distance between the mandrel mill and the extractor is rolled in a conventional retract mandrel mill, that is, a retract mandrel mill in which the distance between the mandrel mill 10 and the extractor 20 is not adjustable, the front end of the tube blank 30 does not reach the extractor 20 after the rolling in the mandrel mill 10 has ended as shown in FIG. 2A.
In such a case, to make the tube blank 30 reach the extractor 20, and also to extract the mandrel bar 11 from the tube blank 30 (to perform stripping), the following three methods are applied.
(1) Regardless of the length required as a product, the tube blank 30 is produced with an extra length such that the length of the tube blank 30 after being rolled in the mandrel mill 10 is longer than the distance between the mandrel mill 10 and the extractor 20. Then, the excess part of the tube blank 30 is cut off in a subsequent step after the mandrel bar 11 is extracted from the tube blank 30 with the extractor.
However, in the method of (1) described above, since it is necessary to produce a tube blank having a length longer than the length needed for a product, there occurs a decrease in the yield of starting material and an excessive energy consumption.
(2) As shown in FIG. 2B, the mandrel bar 11 is forced to retreat while the tube blank 30 is prohibited from moving in the direction opposite to the rolling direction by using the extract fork 14, thereby performing stripping. Thereafter, the tube blank 30 is conveyed to the extractor 20 by conveyor rolls 15.
(3) As shown in FIG. 2C the tube blank 30 after rolling is conveyed by the mandrel bar 11 until when its front end comes into contact with a roll 22 on the entrance side of the extractor 20. Thereafter, the mandrel bar 11 is retreated while the tube blank 30 is rolled by the extractor 20, thereby performing stripping.
In the methods of (2) and (3) described above, it takes time for moving the extract fork 14 from a retreat position to a predetermined position, and for moving the tube blank 30 with the mandrel bar 11. Moreover, the temperature of the tube blank 30 becomes lower while moving. Such a temperature drop causes a thermal contraction of the tube blank 30 so that the stripping becomes difficult to be performed when the overlap (overlapped portion between the tube blank 30 and the mandrel bar 11) is long. In particular, when the tube blank 30 is made of a material that exhibits a large thermal contraction as temperature decreases (for example, an alloy steel with a Cr content of not less than 10% by mass), the stripping may become impossible.
Therefore, as shown in FIG. 2D it is necessary to shorten the overlap during or after rolling. As a method of shortening the overlap, there is a method of reducing the moving speed of the mandrel bar 11 during rolling to be lower than the moving speed of the tube blank 30. However, reducing the moving speed of the mandrel bar 11 results in an increase in speed difference between the mandrel bar 11 and the tube blank 30 and there arises a problem such that the mandrel bar 11 is more liable to be damaged due to friction with the tube blank 30 during rolling in the mandrel mill 10.